ci: Add CD workflow and configure NuGet publishing

[MCGPPeters]

Problem

The Picea.Mariana package is not published on nuget.org. Unlike picea/glauca and picea/rubens, this repo has no .github/workflows directory at all — no CI or CD pipeline exists.

Packages from picea/picea (2 packages) and picea/abies (7 packages) are already live on nuget.org, but Picea.Mariana is missing.

What's Needed

1. Create CD Workflow (.github/workflows/cd.yml)

A workflow that:

• Triggers on push to main (or tags matching v*)
• Builds and tests the solution
• Packs Picea.Mariana/Picea.Mariana.csproj
• Publishes to NuGet using secrets.NUGET_API_KEY

Reference implementation: picea/glauca cd.yml

2. Configure NUGET_API_KEY Repository Secret

Go to Settings → Secrets and variables → Actions → New repository secret and add:

• Name: NUGET_API_KEY
• Value: NuGet.org API key with push permissions for Picea.Mariana

3. Re-tag or push to trigger publishing

Once the workflow and secret are configured, either:

• Push a new commit to main, or
• Delete and re-create the v0.1.0 tag to trigger the workflow

Context

Part of the Picea org migration (Phase 6.10 — NuGet package verification).